1. Field of the Invention
This invention relates to an apparatus for coding and decoding for error correction employed in a recording and/or reproducing system for an optical disk or the like.
2. Description of Related Art
In a disk apparatus employing an optical disk as a recording medium, such as a player for a compact disc (CD) having concentric tracks on which digital audio data, for example, are recorded as a string of pits, a laser beam is irradiated along the tracks on the disc which is rotationally driven at a constant linear velocity (CLV) by a spindle motor, and changes in the intensity of the reflected light caused by the presence or absence of the pits are detected for reproducing the digital audio data.
During data reproduction by the CD player, an error rate may reach an order of 10.sup.-5. In order to combat this, a cross-interleave Reed-Solomon code (CIRC), which is combined from the Reed-Solomon code, an error correcting code having a high random error correcting ability, and means for converting the burst error into a random error using an interleaving technique, is used for error correction, so that no hindrances are produced under ordinary operating environments.
In an audio data recording system, which uses the CIRC, uses an encoding apparatus 110 arranged and constructed as shown in FIG. 9. Digital audio data are inputted to the encoding apparatus in parallel, with 6 samples, that is 16 2 6 equal 192 bits or 24 bytes, for each of left and right channels, as one unit. These input data are rearranged after the data of the even-numbered samples are delayed by a scrambling circuit by two CIRC units. The input data, rearranged by a scrambling circuit 111, are subjected to encoding of the first error correction code C.sub.2 by a first error code generator 112, so as to be supplied to an interleaving circuit 113 after the parity Q of the Reed-Solomon code of m=8, n=28, k=24 and d=5 is appended thereto. The interleaving circuit 113 applies different amounts of delay to the data such that the maximum amount of delay of 108 frames is applied for interleaving. The parity P of the Reed-Solomon code of m=8, n=32, k=28 and d=5 is appended to the data interleaved by the circuit 113, by way of encoding by the second error correction code C.sub.1 by a second error correction code generator 114. The input data, to which the parities Q and P have been appended in this manner, are processed by an odd number delay circuit 115 where only odd-numbered symbols are further delayed by one symbol and the signs of the parities Q and P are inverted to provide a 32 byte long CIRC encode output which is to be recorded.
In a reproducing system for digital audio data, which uses the above CIRC, a decoding apparatus 120 shown in FIG. 10 is employed. In the decoding apparatus, playback data of the data encoded by the encoding apparatus 110 are supplied to an even-number delay circuit 121 where even-numbered symbols are delayed by one frame, and the signs of the parities Q and P are inverted. Error correction is performed in a first error correction circuit 122 by the second error correction code C.sub.1 on the basis of the parity P. The playback data, thus error corrected by the first error correcting circuit 122, are deinterleaved by a deinterleaving circuit 123 which applies a delay which is an inversion of that applied by the interleaving circuit 113 of the encoding apparatus 110 before being supplied to a second error correcting circuit 124. The second error correcting circuit 124 performs an error correction by a first error correcting code C.sub.2 on the basis of the parity Q. The playback data, thus error-corrected by the first error correcting code C.sub.1 and the second error correcting code C.sub.2, are descrambled by a descrambling circuit 125 by data rearrangement corresponding to that performed by the scrambling circuit 111 in the encoding apparatus.
The above described encoding and decoding apparatus have been arranged as integrated circuits and extensively used as encoder and decoder for CIRC processing.
Meanwhile, there has been prescribed for the compact disc (CD) a data format in which 24 channel bit synchronizing signals, produced as eight to fourteen modulation data or EFM data converted from the 8-bit-per-symbol signals, are converted into 14 channel bit data, 14 channel bit or 1-symbol sub-code, 14 32 channel bit or 32 symbol data per se, such as play data and parity data, and each 3 channel bit margin bits provided between adjacent symbols, totalling 588 channel bits, are arranged as one frame and 98 frames are arranged as a sub-code block.
It is noted that, in a so-called CD-interactive (CD-I) system for the compact disc (CD) for simultaneously recording image data, letter data etc. besides audio data, six modes have been prescribed besides the CD-DA mode as, for example, audio data, as shown in FIG. 11.
Thus, for the CD-DA mode having the sound quality level corresponding to the current 16-bit PCM (pulse code modulation), straight PCM (straight pulse code modulation) with the sampling frequency of 44.1 kHz and the number of quantizing bits of 16, is used. For the A level stereo mode and the A level monaural mode having the sound quality corresponding to the long-playing record, ADPCM (adaptive differential pulse code modulation) with the sampling frequency of 37.8 kHz and the number of quantizing bits of 8, is used. For the B level stereo mode and the B level monaural mode having the sound quality corresponding to FM broadcasting, adaptive PCM or ADPCM having the sampling frequency of 37.8 kHz an the number of quantizing bits of 4, is used. Finally, for the C level stereo mode and the C level monaural mode, having the sound quality corresponding to the AM broadcasting, ADPCM with the sampling frequency of 18.9 kHz and the number of quantizing bits of 4, is used.
That is, referring to FIG. 11, where a rectangle mark in black denotes a sector where data are not recorded, the bit reduction rate for the A level stereo mode is one-half that of the CD-DA mode, so that data are recorded at every other sector, with the playback time for a disc being approximately two hours. For the A level monaural mode, the bit reduction rate is 1/4 and data are recorded at every four sectors, with the playback time for a disc being about four hours. For the B level stereo mode, the bit reduction rate is 1/4 and data are recorded at every four sectors, with the playback time for a disc being about four hours. For the B level stereo mode, the bit reduction rate is 1/8 and data are recorded at every eight sectors, with the playback time for a disc being about eight hours. For the C level stereo mode, the bit reduction rate is 1/8 and data are recorded at every eight sectors, with the playback time for a disc being about eight hours. For the C level monaural mode, the bit reduction rate is 1/16 and data are recorded at every 16 sectors, with the playback time for a disc being about 16 hours.
Meanwhile, in the digital audio data reproducing system employing the above described CIRC, if burst errors beyond the correction capabilities should occur in playback data, the errors are distributed evenly throughout an interleaving length and thereby rendered less manifest. With a CD player handling continuous PCM audio data, error data may be interpolated by near-by data, so that foreign sounds are less likely to be produced. However, if the data are those which lack in correlation between adjacent data, or the compressed audio data according to the above-mentioned CD-I system, it is not possible to correct errors in the playback data by near-by data. Therefore, if burst errors in excess of the correction capabilities are produced in the playback data and the errors are evenly distributed throughout the interleaving length, the errors occur discretely for the interleaving length, that is for a time longer than 14.7 ms for the CD-DA mode, so that more data than the errors per se become unusable.